The best-known supplement that throws the kitchen sink at colds, Airborne contains vitamins (A, C, E), minerals (zinc, selenium, magnesium) and echinacea, ginger and other herbs. Back in 2008 the Federal Trade Commission accused the company of making unproven claims about curing and preventing colds and flu; it had to pay a $30 million settlement. Now the ads and packages just say Airborne “supports” the immune system. Some ingredients in Airborne have been tested, with inconsistent results. No clinical trials testing the specific formulas have been published. Airborne and similar formulas are a waste of money.
Lab research suggests that this herbal remedy, usually Echinacea purpurea, can stimulate the immune system and have direct antiviral and anti-inflammatory effects. But human studies on echinacea’s effect on colds or immunity have had inconsistent results. Commercial preparations vary widely in the species and the parts of the plants used, making it hard to compare results. Several large well-designed studies found that echinacea was not better than a placebo at preventing colds or reducing their severity. In short, the claims about echinacea for colds have yet to be supported by solid research.
Despite a common belief that garlic can prevent colds, there has been remarkably little research on humans to see if it actually does. For instance, a study in Clinical Nutrition found that an aged garlic extract taken for three months did not reduce the incidence of colds or flu, but did reduce their severity somewhat when they did occur. But the bottom line is that garlic is no more likely to keep away colds than to repel vampires, unless you eat it raw and the smell makes cold sufferers stay away from you.
Like echinacea, ginseng can affect certain aspects of the immune system, though it’s not clear what practical significance this has. A few preliminary studies suggest that Cold-fX, a patented standardized extract of North American ginseng, may help reduce the frequency and severity of colds when taken twice daily throughout the winter, a claim allowed by Health Canada, which functions like our FDA. There’s no evidence that it can provide relief once you have symptoms (despite claims on the company's website). In the U.S., Cold-fX is available only online. At about $30 a month, it’s not worth it.
These supplements contain “friendly” bacteria that are supposed to strengthen immunity, among other proposed benefits. But studies on whether they can curb colds and other respiratory infections have been inconsistent. One problem is that supplements use countless different strains and doses. The Cochrane Collaboration, which evaluates medical research, has concluded that probiotics may help prevent acute respiratory infections, though there were limitations in the studies and no data for older people. We don’t recommend probiotic supplements for cold prevention.
Vitamin C gained popularity back in the 1970s when Linus Pauling claimed it could prevent and alleviate colds. However, numerous studies since then have failed to confirm any benefit. According to a Cochrane Collaboration review, vitamin C supplements do not prevent colds, except perhaps in people exposed to severe physical stress, such as marathon runners and skiers. And research on the vitamin’s potential role in reducing the severity and/or duration of cold symptoms when taken at their onset has yielded mixed results. The tide has turned against vitamin C. If there were a significant benefit, it wouldn’t be so hard to prove.
Some experts believe that vitamin D can help protect against respiratory infections, in part because it plays key roles in the immune system. But the few clinical trials that have been done had mostly disappointing results. For instance, a study from Winthrop Hospital in Mineola, New York, found that 2,000 IU of vitamin D taken daily for 12 weeks did not reduce the risk of upper respiratory infections. And in a study from New Zealand, monthly megadoses of D (100,000 IU), taken for 18 months, also did not reduce the risk. There are good reasons to take vitamin D supplements, notably for bone health, but not for cold prevention.
This mineral is also essential for immunity. In lab studies, large amounts of zinc can block cold viruses from adhering to the nasal lining and/or replicating themselves. A Cochrane Collaboration review concluded that, compared to a placebo, zinc lozenges can shorten colds by a day and reduce their severity, particularly if started the first day of symptoms. Another research review came to similar conclusions. But there is no good research showing zinc prevents colds. Due to possible side effects (nausea, diarrhea, cramps), the Cochrane report said zinc lozenges, taken the first day of symptoms, are "advised with caution."
There’s no convincing evidence that any supplement can prevent or treat colds. “Cold remedies,” including many over-the-counter drugs, may well make you feel better, since they have a strong placebo effect. That is, if you expect or hope that a remedy is going to help, there’s a fair chance it will, whether it contains vitamins, herbs or just plain old sugar. And, of course, remedies may seem to work because colds go away on their own. Though we don’t recommend them, it probably can’t hurt to take such products when you feel a cold coming on, but taking them throughout cold season, as is sometimes recommended, increases the risk of adverse effects.